Most theories on how the nervous system functions depend heavily on the existence and properties of cell to cell contact known as synapses. For this reason, the study of synapses has been a focal point for neuroscience research for many decades.
Because of its accessibility to biochemical and electrophysiological techniques, and because of its elegant, well defined structure, the neuromuscular synapse (also known as the neuromuscular junction), which occurs at the point of nerve to muscle contact, is one of the most studied and best understood synapses. At the neuromuscular junction, the nerve cell releases a chemical neurotransmitter, acetylcholine, which binds to nicotinic acetylcholine receptor proteins located on post-synaptic muscle cells. The binding of acetylcholine results in a conformational change in the nicotinic acetylcholine receptor protein. This change is manifested by the opening of a transmembrane channel in the receptor which is permeable to cations. The resulting influx of cations depolarizes the muscle and ultimately leads to muscle contraction.
Biological and structural studies have shown that the nicotinic acetylcholine receptor in muscle is a glycoprotein composed of five subunits with the stoichiometry ααβλδ (alpha-alpha-beta-gamma-delta). From these same studies, it is known that each of the subunits has a mass of about 50–60 kilodaltons and is encoded by a separate gene. In vitro reconstitution experiments have shown that this ααβλδ complex is a functional receptor containing both ligand binding sites and a ligand-gated transmembrane channel. (For a review, see Karlin, et al., 1986 and McCarthy, et al., 1986.)
It is now known that a variety of neurotransmitters and neurotransmitter receptors exist in the central and peripheral nervous systems. Despite this knowledge, there is still little understanding of the diversity of receptors for a particular neurotransmitter, or of how this diversity might generate different responses to a given neurotransmitter, or to other modulating ligands, in different regions of the brain. On a larger scale, there is little appreciation of how the use of a particular synapse makes it more or less efficient, or how long-term changes in neuronal circuits might be accomplished by the modification of synapses.
An understanding of the molecular mechanisms involved in neurotransmission in the central nervous system is limited by the complexity of the system. The cells are small, have extensive processes, and often have thousands of synapses deriving from inputs from many different parts of the brain. In addition, the actual number of neurotransmitter receptors is low, making their purification difficult, even under the best of circumstances. Consequently, neither cellular nor biochemical approaches to studying neurotransmission in the central nervous system has been particularly fruitful. This is unfortunate because it is quite probable that the treatment of dementia, Alzheimer's disease and other forms of mental illness will involve modification of synaptic transmission with specific drugs.
Nicotinic acetylcholine receptors found at the vertebrate neuromuscular junction, in vertebrate sympathetic ganglia and in the vertebrate central nervous system can be distinguished pharmacologically on the basis of ligands that open or block the ion channel. For example, the elapid α-neurotoxins that block activation of nicotinic acetylcholine receptors at the neuromuscular junction do not block activation of neuronal nicotinic acetylcholine receptors found on several different cell lines.
To gain access to the neuronal acetylcholine receptors, traditional biochemical and neurophysiological methods have been abandoned in favor of the newer methods of molecular biology. More specifically, using molecular cloning techniques, our group first isolated complementary DNA clones encoding the acetylcholine receptor expressed in the Torpedo fish electric organ, a highly enriched source of receptor (see Ballivet, et al., 1983 and Patrick, et al., 1983) were isolated. The cDNA clones isolated from the fish electric organ were then used in nucleic acid hybridization experiments to obtain cDNA and genomic clones for the subunits of the acetylcholine receptor expressed in mouse skeletal muscle.
The availability of cDNA clones encoding the muscle nicotinic receptor made it possible to extend these studies in the important direction of neuronal receptors. More specifically, based on the assumption that the neuronal nicotinic receptors are evolutionarily related to the muscle receptors, and that this relationship will be reflected at the genetic level by nucleotide sequence homology, the cDNA clones encoding the muscle nicotinic receptor were used to screen rat and mouse cDNA and genomic libraries for related neuronal mRNAs or genes. This method has resulted in the isolation of several neuronal cDNA clones that have significant sequence homology with the muscle acetylcholine clones. Clones, which encode the neuronal nicotinic acetylcholine receptor subunit proteins referred to as alpha2, alpha3, alpha4, alpha5, and beta2, beta3 and beta4, are disclosed in the present specification.
These neuronal clones encode a family of acetylcholine receptors having unique pharmacological properties. In this regard, the realization that the nicotinic acetylcholine receptors are much more diverse than previously expected offers an opportunity for a level of pharmaceutical intervention and a chance to design new drugs that affect specific receptor subunits. Such subtypes make it possible to observe the effect of a drug substance on a particular subtype. Information derived from these observations will allow the development of new drugs that are more specific, and therefore have fewer unwanted side effects.
In addition, the availability of these neuronal receptors makes it possible to perform initial in vitro screening of the drug substance. While it is true that the drug eventually has to work in the whole animal, it is probable that useful drugs are being missed because conventional screening is limited to average composite effects. Consequently, the ability to screen drug substances in vitro on a specific receptor subtype(s) is likely to be more informative than merely screening the drug substance in whole animals.
Both the receptor subunit genes and proteins of the present invention can be used for drug design and screening. For example, the cDNA clones encoding the alpha2 through alpha5 and beta2 through beta4 receptor subunits can be transcribed in vitro to produce mRNA. This mRNA, either from a single subunit clone or from a combination of clones, can then be injected into oocytes where the mRNA will direct the synthesis of the receptor molecule(s). Alternatively, the clones may be placed downstream from appropriate gene regulatory elements and inserted into the genome of eukaryotic cells. This will result in transformed cell lines expressing a specific receptor subtype, or specific combinations of subtypes. The derived cell lines can then be produced in quantity for reproducible quantitative analysis of the effects of drugs on receptor function.